This invention relates to lasers and, more specifically, to multi-wavelength lasers such as may be suitable for dense wavelength division multiplexed (DWDM) applications. It also pertains to methods by which such lasers operate.
Wavelength division multiplexing, or WDM, is a known method for increasing the capacity of a fiber optic communication system. WDM increases capacity by permitting a single optical fiber to carry multiple optical carrier signals having differing wavelengths. After transmission, at the receiving end, the multiple wavelengths are demultiplexed to separate the signals.
The concepts behind WDM have been extended to use a set of closely-spaced wavelengths in the 1550 nm window. The International Telecommunication Union (ITU) has proposed the use of a grouping or grid of wavelengths in this window. As presently configured, the channels are anchored to a reference at 193.10 THz and equally spaced in frequency, the closely spaced grids having channels 100 GHz or 50 GHz apart. In the wavelength range between 1528.77 nm and 1560.61 nm, the ITU 100 GHz grid comprises 41 channels. This method of WDM is known as dense wavelength division multiplexing, or DWDM.
Communication systems to implement this scheme thus must have access to emissions at each of the grid wavelengths. This could be accomplished by having a multitude of different laser sources, each having an emission wavelength corresponding to a respective one of the grid wavelengths. It would be far more convenient, cost-effective and efficient, however, to have the capability of producing different wavelengths without having to increase correspondingly the number of different laser sources. A single laser source capable of having multiple emission wavelengths at ITU grid wavelengths is therefore desirable. This type of laser source would also prove advantageous in sparing and hot sparing configurations for DWDM, and would allow for reconfigurable DWDM optical communication networks and network elements. Such an apparatus should be capable of producing signals across the widest possible wavelength range, and provide consistent optical output power across its entire wavelength operating range.
These and other ends are met in the present invention through provision of a laser apparatus comprising a gain module which is pumped by applied pump radiation, the pump radiation exciting the gain module and thereby achieving lasing action. The laser apparatus also has a control module, with the control module including a periodic filter arranged to receive optical energy from the gain module and having relatively higher transmissivity in certain predefined frequency bandwidths to define a first set of frequency passbands, and a resonant filter arranged to receive optical energy from the periodic filter and defining a second set of frequency passbands, the second set of frequency passbands being a subset of the first set of frequency passbands.
The laser apparatus is preferably configured as a ring laser resonator.
The control module may further comprise an optical circulator with the periodic filter and the resonant filter being coupled using the optical circulator. The first set of frequency passbands may correspond to International Telecommunication Union frequency grid recommendations.
The periodic filter may be a transmission filter or a reflection filter. If it is a transmission filter, it may be any one of a number of specific types of transmission filters including a fiber Fabry-Pxc3xa9rot micro-etalon transmission filter or a fiber-coupled Fabry-Pxc3xa9rot micro-etalon transmission filter. If it is a reflection filter, it may be any one of a number of specific types of reflection filters including a sampled fiber Bragg grating or a set of sampled fiber Bragg gratings. The periodic filter may also be tunable.
The laser apparatus may also include means for ensuring unidirectional laser oscillation, which may be an optical isolator, and an optical gain-flattening filter for obtaining an approximately constant laser output power for specified wavelengths of operation of the laser resonator. The optical gain-flattening filter may have a wavelength-dependent loss curve which compensates the wavelength-dependent gain curve of the gain module and may be made up of a set of long-period fiber gratings. It is also possible that the optical gain-flattening filter may be incorporated into the gain module. The laser apparatus may also include at least one polarization controller.
The periodic filter may have passbands spaced apart at a frequency spacing of 200 GHz or a sub-multiple of 200 GHz. The center frequency of at least one passband of the first set of frequency passbands may be maintained referenced to a predetermined frequency so as to obtain a laser output with wavelengths according to the International Telecommunication Union frequency grid recommendations. The resonant filter then has a subset of frequency passbands selected from the set of frequency passbands defined by the periodic filter. The resonant filter passband bandwidths may be sufficiently narrow that within each resonant filter passband laser oscillation occurs at wavelengths within a single passband of the passbands of the periodic filter. In another embodiment, each of one or more of the resonant filter passbands may enclose more than one of the passbands of the periodic filter, allowing multiple-wavelength laser oscillation with wavelengths within adjacent passbands of the periodic filter.
The control module may also include an optical isolator, and may further include a wavelength reference control module for locking a spectral response of the periodic filter to the first set of passbands. The wavelength reference control module may include a coupler, optically coupled to the other elements in the resonator, for extracting a small fraction of laser radiation from the laser cavity, a photodetector, and a wavelength reference filter for coupling a narrow-band optical signal from the coupler to the photodetector to provide a stable wavelength reference. The wavelength reference filter may be a reflective wavelength reference filter, and, if it is, it may be made up of a temperature-compensated fiber Bragg grating.
The control module may further include a spectral selective transmission module which may be made up of a set of fiber-fused Mach-Zehnder interleavers.
At least one of the periodic filter and the resonant filter may be only partially reflective to provide a means for extracting optical output energy without the use of a separate output coupler. Also, at least one of the resonant filter and the periodic filter may include means for accessing individual oscillation wavelengths of the laser. The means for accessing individual oscillation wavelengths of laser may be separated output optical fibers. A separate laser output power control module may be inserted in-line with each of the output optical fibers. The means for accessing individual oscillation wavelengths of laser may also be made up of a spectral selective transmission module and a set of filters. The spectral selective transmission module may be comprised of a set of fiber-fused Mach-Zehnder interleavers and the set of filters may be comprised of a set of fiber Bragg gratings.
At least one of the periodic filter and resonat filter may have a wavelength-dependent envelope curve, which compensates a wavelength-dependent gain curve of the gain module.
The laser apparatus may also include a laser output power control module, inserted in-line with an output of the laser, for setting and maintaining a predetermined level for optical output power of the lasers. The laser output power control module may be made up of a coupler to extract a fraction of laser optical output power, a calibrated photodetector optically coupled to the coupler and producing an electric reference signal, a loop control unit, arranged to receive the electric reference signal, for generating an control signal based on the electric reference signal, and an in-line variable optical attenuator arranged to receive the control signal, for controlling the laser optical output power based on the control signal.
The invention also resides in a method of generating a multiple wavelength laser output at several of a set of discrete frequencies, the method comprising the steps of providing pump energy to a gain medium in a laser cavity to excite laser resonances, filtering the laser resonances using a periodic filter to limit possible lasing frequencies to the frequencies in the set; and filtering the laser resonances using a resonant filter to limit possible lasing frequencies to a subset of the frequencies in the set.